This invention relates to window assemblies subjected to extreme heat such as the infrared seeker window in an interceptor missile.
High speed interceptor missiles often incorporate infrared radiation seeker technology to aid in target discrimination. A window assembly formed in the body of the missile is placed in optical communication with the infrared seeker subsystem so that it can receive and analyze infrared radiation emitted by the target. In some designs, when the interceptor missile closes in on a target in flight, a protective cover over the window assembly is blown off the missile, the infrared seeker receives infrared radiation emitted by the target, and, in response, the trajectory of the interceptor missile is adjusted to properly intercept the target.
One important design consideration of the window assembly is the frictional heating caused by the high velocity flow of air over the outer surface of window assembly. If not addressed, this heating can cause destructive thermal shocks, optical distortion, and/or cause the window itself to emit infrared radiation which interferes with the image received by the infrared sensor on board the interceptor missile.
Accordingly, two prior art methods have been developed in an attempt to cool the window assembly from the frictional heating effects of the air stream flowing over it. In one method, helium gas is caused to flow along the outside of the window between the exterior surface of the window and the boundary layer. This method, called xe2x80x9cexternal film coolingxe2x80x9d suffers from the disadvantages that a large quantity of cooling gas must be stored on board the interceptor missile, special design considerations must be employed to insure a uniform boundary layer, and the associated valves, feedback mechanisms, and the complexity of such a system results in a costly system prone to failure.
The cooling effectiveness of the stream of gas over the outer surface of the window can be adversely impacted by changes in attitude and interactions between the divert thrusters of the missile and the air stream. In addition, the turbulent interaction between the atmospheric and coolant streams can degrade image quality, which limits the choice of cooling fluids to a lightweight gas, such as helium and precludes the use of other cooling gas design choices. The impact of this is to constrain external film cooled systems to the use a cooling gas which limits the maximum packaging efficiency.
In another prior art approach, called xe2x80x9cinternal liquid coolingxe2x80x9d, internal channels are formed within the window to carry a liquid coolant. Since the liquid coolant is opaque to infrared radiation, however, the internal cooling channels must be made relatively narrow and widely spaced in order to transmit sufficient infrared radiation through the window. In other words, only the infrared radiation impinging on the window in the areas of the window which are not cooled by the internal liquid cooling channels can be imaged and thus the active area of the window is limited by the space taken up by the cooling channels. Moreover, significant temperature gradients created between and along the cooling channels produce a laterally non-uniform index of refraction which degrades the infrared radiation image. Also, defraction of signals from targets or the sun by the cooling channels can cause false targets in the field of view of the window.
It is therefore an object of this invention to provide an internal fluid cooled window assembly.
It is a further object of this invention to provide such a window assembly which can be effectively cooled without using as much gas as an externally cooled window assembly.
It is a further object of this invention to provide such a window assembly which can be effectively cooled without adversely affecting the optical characteristics of the window.
It is a further object of this invention to provide such a window assembly which does not require special design considerations employed to ensure a uniform boundary layer.
It is a further object of this invention to provide such a window assembly which does not require complex valves and feedback mechanisms thus resulting in a less costly design.
It is a further object of this invention to provide such a window assembly which is effectively cooled irrespective of changes in the attitude of the missile and interactions between the divert thrusters of the missile and the air stream flowing over the window.
It is a further object of this invention to provide such a window assembly which can be cooled using a number of different kinds of gases to improve the packaging efficiency.
It is a further object of this invention to provide such a window assembly which utilizes an internal cooling gas transparent to infrared radiation.
It is a further object of this invention to provide such a window assembly which has wide cooling channels separated by narrow spacer elements to reduce or eliminate temperature gradients created between and along the cooling channels.
It is a further object of this invention to provide such a window assembly which does not result in false targets in the field of view of the window assembly.
It is a further object of this invention to provide such a window assembly which is effectively cooled without degrading image quality.
It is a further object of this invention to provide such a window assembly which meets or exceeds the mechanical loading and thermal mechanical shock requirements for advanced interceptor missiles.
It is a further object of this invention to provide such a window assembly which requires less cooling volume and simpler gas flow controls.
It is a further object of this invention to provide such a window assembly which minimizes lateral temperature gradients and the resulting spatially independent phase errors.
It is a further object of this invention to provide a window assembly which can be used in conjunction with any high temperature vessel.
The invention results from the realization that a missile window assembly can be effectively cooled without using as much gas as an externally cooled window and without disrupting the optical characteristics of the window as is the case with internal liquid cooled windows by including wide cooling channels separated by narrow spacer elements between a strong thick inner window and a thin outer window and by utilizing a fluid in the cooling channels such as a gas which is transparent to infrared radiation.
This invention features an internal fluid cooled window assembly comprising an inner window, an outer window, and a support subsystem between the inner window and the outer window defining at least one transparent fluid flow channel between the inner and outer windows for cooling the outer window without adversely affecting the optical properties of either window.
The inner window typically has a thickness substantially greater than the thickness of the outer window and the support subsystem preferably includes a plurality of longitudinally running spacer elements between the inner and outer windows, each pair of adjacent spacer elements defining a cooling channel therebetween. In one embodiment, each spacer element is made of two different materials and preferably the materials of the spacer elements in combination have a thermal conductivity which matches the convective heat transfer rate of the fluid flowing in the channels.
For use in conjunction with interceptor missiles, the fluid is preferably a gas such as nitrogen, helium, argon, or sulfur hexaflouride all of which are transparent to infrared radiation. In other environments, the fluid may be a liquid which includes water.
The inner and outer windows are preferably made of a material such as aluminum oxidynitride, yttria, aluminum oxide, zinc sulfide, silicon, gallium phosphide, or diamond. Two design considerations are that each cooling channel between the inner and outer windows should have a cross sectional area sufficient to prevent sonic flow velocities of the fluid flowing therein and the support subsystem preferably defines a plurality of flow channels the combined area of which is substantially greater than area occupied by the support subsystem.
An interceptor missile in accordance with this invention includes an infrared radiation detection subsystem and a window assembly in the hull of the missile optically coupled to the infrared radiation detection subsystem. The window assembly includes an inner window, and outer window, and a support subsystem between the inner and the outer windows defining a plurality of infrared transparent gas flow cooling channels between the inner and outer windows. A source of gas is coupled to the cooling channels for cooling the outer window.